1. Field of the Invention
This invention relates, in general, to a process for preparing catalysts and, specifically, to a process for preparing catalysts for polymerization of olefins.
2. Description of Related Art
Olefins, especially propylene, may be polymerized to form polyolefins in crystalline and amorphous forms. Isotactic and syndiotactic are crystalline and atactic is amorphous. Isotactic olefins are typically described as having groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, i.e., the groups are all above or below the plane. This structure may be represented as follows: ##STR1##
Isotactic polypropylene is capable of being a highly crystalline polymer with a high melting point and other desirable physical properties that are considerably different from the polymer in an amorphous (noncrystalline) state.
A syndiotactic polymers are those in which the groups attached to the tertiary carbon atoms of successive monomeric units lie on alternate sides of the plane of the polymer. Syndiotactic structure is represented as follows: ##STR2##
Atactic polymer shows no regular order of repeating unit configurations. In commercial applications, a certain percentage of atactic polymer is typically produced with the crystalline forms.
Polymerization of olefins is primarily with Zeigler-Natta catalysts. One family of Zeigler-Natta catalysts is Group IVB metallocene compounds with methylaluminoxane as a cocatalyst. It has been demonstrated that Zeigler-Natta catalysts for olefin polymerization can be formed by combining a Group IVB metallocene with an ionic compound. ##STR3## Cp*--pentamethylcyclopentadienyl M--Group IVB metal
R--alkyl PA0 L--ligand PA0 [C]--cation PA0 [A]--anion PA0 One-Electron Oxidation--This method is illustrated in "Ethylene Polymerization by a Cationic Dicyclopentadienylzirconium(IV) Alkyl Complex", R. F. Jordan, C. S. Bajgur, R. Willett, B. Scott, J. Am. Chem. Soc., p. 7410-7411, Vol. 108 (1986). These early examples used a Lewis base to make the metal cation less electrophilic and [BPh.sub.4 ]-- was the anion where Ph is C.sub.6 H.sub.5. The reaction occurred in a coordinating solvent. These catalysts were usually of low activity. PA0 Protonation--This method is illustrated by "Synthesis and Insertion Reactions of Cationic Alkylbis(cyclopentadienyl)titanium Complexes", M. Bochmann, L. M. Wilson, J. Chem. Soc. Commun., p. 1610-1611, (1986); PA0 "Cationic Alkylbis(cyclopentadienyl)titanium Complexes", M. Bochmann, L. Wilson, Organometallics, p. 2556-2563, Vol. 6, (1987); Insertion Reactions of Nitriles in Cationic Alkylbis(cyclopentadienyl)titanium Complexes, M. Bochmann, L. Wilson, Organometallics, p. 1147-1154, Vol. 7 (1987). PA0 Ligand abstraction--The ligand abstraction method is illustrated in "Multiple Metal-Carbon Bonds", R. R. Schrock, P. R. Sharp, J. Am. Chem. Soc., p.2389-2399, Vol. 100, No, 8 (Apr. 2, 1978).
The resulting compound is a metallocene cation which acts as a catalyst. The cation [C] of the ionic compound reacts with the metallocene to generate an ion pair. The anion, [A], is not coordinated or is only loosely coordinated with the cationic metallocene.
The following methods have been used to produce the above reaction:
European Patent Application 0-277-003 relates to a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with a compound having a cation capable of donating a proton and an anion having a plurality of boron atoms. For example, the following reaction illustrates the invention:
______________________________________ bis(pentamethylcyclopentadienyl)zirconium dimethyl + 7,8- dicarbaundecaborane.fwdarw.bis(pentamethylcyclopentadienyl)(methyl- dodecahydrido-7,8-dicarbaudecaborato)zirconium + CH.sub.4 ______________________________________
European Patent Application 0-277-004 also relates to a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with an ionic compound having a cation which will irreversibly react with a ligand on the metal compound and an anion having a plurality of lipophilic radicals around a metal or metalloid ion. For example, the following reaction illustrates the invention:
______________________________________ tri(n-butyl)ammonium tetra(pentafluorophenyl)boron + bis(cyclo- pentadienyl)zirconium dimethyl.fwdarw.[Cp.sub.2 ZrMe][BPh.sub.4 ] + CH.sub.4 + tri(n-butyl)N ______________________________________
A by-product of the protonation reaction is a Lewis base (amine) some of which can coordinate to the cations and thus inhibit catalyst activity. Starting materials must be chosen carefully to avoid generating particular amines which are catalyst poisons. In addition, the catalyst and the polymer produced with this catalyst contains undesirable and toxic residual amines.
In most known processes, methylaluminoxane (MAO) is added with the metallocene compound to act as a cocatalyst. The function of MAO is to initiate the alkylation and promote ionization of the metallocene compound. The cocatalyst is a scavenging agent which reduces the poisons which decrease catalyst activity. Present known processes require a large excess of MAO to be added. MAO has the disadvantage of being relatively expensive. This results in high costs for the catalyst system.